The present invention relates to brake control systems for motor vehicles, which allow wheel anti-lock brake control and stabilizing control such as wheel traction control or vehicle motion control.
One of the conventionally proposed brake control systems which allow stabilizing control is known in U.S. Pat. No. 5,015,043 issued to Resch. This brake control system includes two hydraulic control circuits between a master cylinder as a fluid-pressure source and wheel cylinders of driving and driven wheels. The hydraulic control circuit inputs a fluid pressure produced in the master cylinder and a discharge pressure of a main pump and outputs them to the wheel cylinders. The circuit includes pressure increasing and decreasing valves to control its output pressure with respect to a wheel-cylinder pressure, and an auxiliary pump in series with the main pump to suck brake fluid on the master-cylinder side and discharging it to the suction side of the main pump. Moreover, the circuit includes a normally-open OUT-side gate valve between the master cylinder and the circuit, and a normally-closed IN-side gate valve on the suction side of the auxiliary pump.
During anti-lock brake control, the brake control system keeps the OUT-side gate valve open and the IN-side gate valve closed, and drives a motor to supply the discharge pressure of the main pump to the hydraulic control circuits. And the system actuates the inlet and outlet valves to control the wheel-cylinder pressure. Thus, the wheel skid rate is maintained within a predetermined range, preventing wheel lock or skid. At that time, the auxiliary pump, though driven by the motor, cannot suck and discharge brake fluid due to the IN-side gate valve being closed, failing to fulfill a function of supplying brake fluid.
During stabilizing control, the brace control system closes the OUT-side gate valve and opens the IN-side gate valve, and drive the motor of the auxiliary pump to supply the fluid pressure of the master cylinder to the main pump which supplies to the hydraulic control circuit the fluid pressure derived from the auxiliary pump upon start-up, then its discharge pressure obtained by sucking brake fluid within a reservoir. And the system actuates the inlet and outlet valves to supply a brake-fluid pressure to the wheel cylinders, producing a braking force to control wheel torque or stabilize a vehicle posture by producing a yaw moment in a vehicle.
However, the known brake control system produces the following problem. During stabilizing control, the system opens the normally-closed IN-side gate valve and closes the normally-open OUT-side gate valve, and drives the main and auxiliary pumps to control the wheel-cylinder pressure. When finishing stabilizing control, stopping of the motors and switching of the gate valves to the normal state are carried out simultaneously.
In that case, after receiving stop signals, the motors continue to rotate for a period of time by inertia, so that the main and auxiliary pumps also continue to operate for a period of time. Therefore, brake fluid within a section downstream of the IN-side gate valve as closed continue to be sucked, producing a negative pressure between the IN-side gate valve and a suction valve of the auxiliary pump. Upon subsequent stabilizing control, due to this negative pressure, the auxiliary pump cannot suck brake fluid immediately after its starting, failing to fulfil a function of supplying brake fluid. Thus, satisfactory pressure rise performance cannot be obtained upon starting of the main pump, resulting in insufficient control responsivity. Achievement of sufficient control responsivity needs determination of motor and pump capacities in consideration of the negative pressure, causing an inevitable increase in the motor and pump capacities, resulting in an increase in system size and manufacturing cost.
It is, therefore, an object of the present invention to provide brake control systems for motor vehicles, which allow wheel anti-lock brake control and stabilizing control, with sufficient control responsivity and reduced system size and manufacturing cost.